Most people do not start out as heedless adventurers hell-bent on the road to destruction.

From Once a Warrior King by David Donovan.  Page 191.  

Most people do not start out as heedless adventurers hell-bent on the road to destruction. It’s just that danger is like most things in life: you can get used to it. After a while you stop paying quite so much attention to it, then you ignore it, and finally you are contemptuous of it. You might even be so audacious as to taunt it on occasion, just to see if it can be pricked to a response. But the worst thing of all is to think that it is no longer worthy of your attention. That’s when you start doing stupid things and making stupid decisions. I am as guilty as anyone on all counts.

Like most everyone else entering a combat zone, I spent my first weeks nervous and scared. I swore to myself that I would obey all the rules, that I was never going to be caught without my helmet and flack vest, and that I would never, absolutely never, take an unnecessary risk. Well, it is hard to be always disciplined and constantly on the alert when danger surrounds you twenty-four hours a day. It grinds at you and wears you down, and you start to lose that edge - units are allowed to move without flank security, night positions are taken without going to the trouble of digging in, commanders ignore commonsense precautions, and the troops are allowed to be inattentive to common dangers. God knows how many men were killed in Vietnam not by some inevitable act of war, but by some act of carelessness by men practically inviting death.

I’ve done some pretty dumb things on military operations myself, things that would make the referees at the Infantry School choke - and I wouldn’t blame them - but I was always lucky, and I managed to get out of rough spots one way or another. I was lucky in combat and I was lucky in other places, too.